Dielectric window for reactor like ICRF vacuum transmission line

The dielectric window of a 35–60 MHz ion cyclotron resonance frequency (ICRF) transmission line is designed for reactor conditions in due consideration of irradiation, dielectric heating, fabrication issues, and remote handling. Electric field, temperature distribution, and thermal stresses are evaluated for beryllia and alumina dielectrics using finite element codes. The analysis is made for two annular ceramic septa joining coaxial water-cooled conductors having maximum operating 50 kV peak rf voltage and 30 Ω characteristic impedance. For unfavourable irradiation conditions (>1023 n/m2 neutron fluence), alumina is found to be heated excessively to about 1000 °C with unacceptable stresses. For moderately irradiated beryllia (<1022 n/m2) or for unirradiated alumina of specific grades (tan δ comparable or less than 5×10−4), the temperature and the thermal stresses are found to stay acceptable provided niobium or titanium is used as a conductor. Beryllia (BeO) is chosen as a candidate for the window ceramic because of its better thermal conductivity and smaller thermal expansion mismatch with the conductor material helping also the manufacturing process. The characteristics for the ceramic/metal joints are estimated for candidate conductor and ceramic (alumina) materials. Vacuum brazing using active filler materials provides sufficiently good conditions for heat conduction across the joint, and the joints appear to be tight enough. Suitable window location on the transmission line is investigated by calculating neutron fluences at the window using an experimental reactor like configuration with 2500 h total burn time (1500 MW)